Hum,
it seems that the sandstone impact site is the source of the Libyan desert glass, which has already been dated to 28.5 million years ago.
“The terrain in which the crater resides is composed of 100 million year-old sandstone – the same material that lies under much of the eastern Sahara. The researchers hope that field investigations and samples of the host rock will help in determining the exact age of the crater and its surroundings.”
There was no other exposed sandstone in the area...
http://www.bu.edu/phpbin/news/releas...ay.php?id=1073